As I commented some months back, in what appears to be a major program of replacing gas boilers in council places a year or two back (as evidenced by all the council properties suddenly growing copper vines up all the exterior walls), there's not one bit of sleeving used anywhere -- all the pipes are mortared directly in (very neatly so, it has to be said).
When I put my heating in some years back, I had only ever seen plastic pipe used as sleeves. For the 28mm pipe, I used 40mm plastic (or might have been 32mm, but I doubt that would have fitted). For the 22m and 15mm pipework, I sleeved it in next size up copper. I'd never seen this done, and I did wonder if someone might claim it was non-conformant because the sleeve might corrode in the wall. Where I had 15mm water pipes going through walls, I sleeved those in 20mm plastic electrical conduit. I was expecting the water pipes to slide in the sleeving due to expansion and contraction, but with the gas pipework sleeved in copper, the gas pipe is fixed so it runs centrally through the sleeve and can't touch (these sleeves are between rooms, so there's no seal either side).
Copper sliding against copper can eventually wear through the pipe. Had a number of aircon systems fail due to this in various commercial premises.
Incidentally, the putty-type seal which British Gas have used in my meter box is reacting nicely with the copper pipe it surrounds. The resulting copper salts have migrated some way into the putty.